当我用的是运营商，为什么只有一个空支票IL代码？ [英] When I use is operator why there is only a null-check in IL code?

问题描述

我想知道是如何是运营商 C＃实施。而我写了一个简单的测试程序（没有特别的，只是为了演示）：

I was wondering how is is operator implemented in C#.And I have written a simple test program (nothing special, just for demonstration purposes):

class Base
{
    public void Display() {  Console.WriteLine("Base"); }
}

class Derived : Base { }

class Program
{
    static void Main(string[] args)
    {
        var d = new Derived();

        if (d is Base)
        {
            var b = (Base) d;
            d.Display();
        }
    }
}

和看着生成的 IL 代码：

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       27 (0x1b)
  .maxstack  2
  .locals init ([0] class ConsoleApplication1.Derived d,
           [1] bool V_1,
           [2] class ConsoleApplication1.Base b)
  IL_0000:  nop
  IL_0001:  newobj     instance void ConsoleApplication1.Derived::.ctor()
  IL_0006:  stloc.0  // set derived (d)
  IL_0007:  ldloc.0 // load derived
  IL_0008:  ldnull // push a null reference
  IL_0009:  ceq   // and compare with d !?
  IL_000b:  stloc.1
  IL_000c:  ldloc.1
  IL_000d:  brtrue.s   IL_001a
  IL_000f:  nop
  IL_0010:  ldloc.0
  IL_0011:  stloc.2
  IL_0012:  ldloc.0
  IL_0013:  callvirt   instance void ConsoleApplication1.Base::Display()
  IL_0018:  nop
  IL_0019:  nop
  IL_001a:  ret
} // end of method Program::Main

当我看<一结束HREF =http://msdn.microsoft.com/en-us/library/system.reflection.emit.opcodes.ldnull%28v=vs.110%29.aspx>文档它说：

推空引用（O型）到计算堆栈。

Pushes a null reference (type O) onto the evaluation stack.

为 ldnull 。 Ofcourse，我没想到在这里看到一个源代码，但我很惊讶，只存在一个空check.I认为这可能与有关编译器优化，因为派生从基础派生所以没有检查有关types.then兼容性我检查出来，看到优化是关闭的。当我打开优化甚至没有的空检查的

for ldnull. Ofcourse, I wasn't expecting to see a source code here, but I'm surprised that there is only a null-check.I thought it may be relevant with compiler optimizations because Derived derives from Base so there is no check the compatibility about the types.then I check out and see that the optimizations are turned off.when I turn on the optimizations there wasn't even null-check.

所以，问题是为什么会出现约产生什么是运营商？为什么我看到的只是一个空检查？是它在某种程度上与相关是运营商，我不能看？

So the question is why there is nothing generated about is operator ? why I see only a null-check ? Is it somehow relevant with is operator and I couldn't see ?

推荐答案

类型 D 是导出，它的类型总是基本或无效即可。这就是为什么非优化的代码只为null检查。

The type of d is Derived, which is always of type Base or null. That's why the non-optimized code only checks for null.

优化的代码，因为优化器知道 D没有做的检查都不为空（因为你分配一个新对象的话）和分配后并没有改变。

The optimized code doesn't do a check at all because the optimizer knows that d is not null (since you assigned a new object to it) and didn't change after the assignment.

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